Traditional sheet metal forming processes generally begin with a drawing operation in which a blank is drawn or stamped to a desired shape. Drawing processes are limited by the depth of draw required to form a panel and the width of the flange that is drawn into the die cavity. As the depth of the cavity increases, the flange of the blank must be wider to provide additional material that may be drawn into the cavity. The stress applied to the blank at the entrance of the cavity should be less than the yield stress of the sheet metal blank. If the yield stress exceeds the yield stress of the sheet metal blank, deformation at the entrance of the die cavity may result in the formation of splits in the material.
The drawing process may be divided into several steps to reduce stress at the entrance of the die cavity. Factors contributing to the stress at the entrance of the die cavity include the width of the flange, the amount of clamping force applied by the blank holder to prevent wrinkling, the coefficient of friction, and the size of the radius at the entrance to the die cavity. If the stress at the entrance of the die exceeds its maximum value after which the strain is localized, and the blank is splitting, it may be necessary to use several draw dies for a deep drawing operation. If the number of dies is increased, substantial expense is incurred to fabricate dies. In addition, if several draw dies are required, additional draw presses must be provided in a press line, or extra positions must be provided in a transfer press.
Other approaches to reducing the stress at the entrance of the die cavity may include the use of hydro-mechanical drawing techniques in which a punch is driven into a blank that is positioned over a die cavity that is filled with a liquid. As the punch engages the blank, a pocket of metal may accumulate around the punch as a result of the resistance of the fluid to be force applied by the punch. Hydro-mechanical drawing operations are not normally effective to form detailed features in a panel. In addition, hydro-mechanical drawing of automotive panels requires hundreds of liters of process water to be pumped into and out of the die cavity as the blank is formed by the punch. Process cycle times are lengthened due to the need to pump the process water in and out of the die cavity.
Another approach to reducing the stress at the entrance of the cavity may include providing a lubricant that reduces the friction between the blank, the die, and the clamping ring at the entrance of the cavity.
Aluminum alloys and advanced high strength steel (AHSS) are more subject to splitting due to stresses applied during a drawing operation. Aluminum and AHSS have reduced formability compared to low carbon steel. Problems relating to providing a multi-step drawing process are increased with aluminum and AHSS alloys. The use of hydro-mechanical drawing processes may increase the formability of draw panels, but is slow due to the need to pump process water into and out of the die cavity.
The above problems are addressed by this disclosure as summarized below.